Reorder documentation to emphasize strongly typed data structures

[HastD]

Most JSON data that people work with in practice has a known structure, so it's suitable for parsing into a user-defined data structure—which tends to result in clearer, more robust code—rather than using serde_json::Value. However, the main README places the "Operating on untyped JSON values" section before the "Parsing JSON as strongly typed data structures", which emphasizes the untyped approach. I suggest switching the order of these sections to encourage new users to take the strongly-typed approach.